1. Field of the Invention
The present invention is related to an uninterruptible power supply (UPS) supporting active loads, and more particularly to a UPS satisfying the hold-up time demanded by active loads.
2. Description of the Related Art
CBEMA (Computer and Business Equipment Association) stipulates that computers and commercial equipment shall be able to withstand power outage or signal transition time for at least 8 milliseconds so that the computers and commercial equipment can sustain basic working power for a while during the course of power outage or signal transition. Usually, computers and commercial equipment are all equipped with power supplies to support their operating power. Hence, the power supplies of computers and commercial equipment shall be able to sustain the operating power thereof for 8 milliseconds after a power failure, so as to smoothly complete the power-off procedure.
With reference to FIG. 8, an active power supply (70) having a time-sustaining function includes a bridge rectifier (71), an active power factor correction (PFC) circuit (72), an output capacitor Cbulk and a DC to DC converter (73).
An input terminal of the bridge rectifier (71) is connected to an AC mains to convert the AC power into a DC power. The active PFC circuit (72) has an input terminal and an output terminal. The input terminal of the active PFC circuit (72) is connected with an output terminal of the bridge rectifier (71) to perform power factor correction of the DC power outputted from the bridge rectifier (71) so as to make the phases of the voltage and current of the AC MAINS consistent. The output capacitor Cbulk is connected to an output terminal of the active PFC circuit (72). The DC to DC converter (73) has an input terminal and an output terminal, and the input terminal is connected with the Cbulk to acquire the DC power and output the DC power after converting it into a low voltage of DC power.
The so-called active power supply (70) has its name simply after the adopted active PFC circuit (72). The active PFC circuit (72) has a time-sustaining circuit (not shown). With reference to FIG. 9A, which illustrates the function of the time-sustaining circuit, the vertical axis VAC vs. the horizontal time axis shows the waveform of the power of the AC mains to the input terminal of the active power supply (70). When VAC is cut off, the time-sustaining circuit supplies the DC power stored therein to the output capacitor Cbulk to delay the time that the voltage of the DC power VBH of the output capacitor Cbulk drops to a low voltage threshold value VBL. With reference to FIG. 9B, the vertical axis VAC vs. the horizontal time axis B shows the hold-up time TH, which is the time that the maximum voltage VBH of the output capacitor Cbulk drops to the low voltage threshold value VBL.
To keep computers and commercial equipment constantly running under a power outage of the AC mains, UPSs are supplemented for that role. In other words, UPSs are connected between the AC mains and power supplies of computers and commercial equipment. When the AC mains normally supplies power, the UPSs store power. When the AC mains is out, the power stored in the UPSs is converted into an AC power to continuously supply power to the computers and the commercial equipment. Hence, from the point of the power supplies, the AC power of the input terminal is always on so as to keep supplying power to the computers and commercial equipment. However, current UPSs fail to support all types of power supply devices, especially active power supply devices. The reason is explained as follows:
With reference to FIG. 10, a UPS (80) whose output terminal is connected with an input terminal of an active power supply (70), has a transformer (82), a switch circuit (81), a controller (85), a charger (83) and a battery set (84). The transformer has a primary side and a secondary side, in which the secondary side is coupled to an AC mains (AC/IN). The switch circuit (81) is a push-pull converter having two active switches G1 and G2 respectively connected with two windings of the primary side of the transformer (82). The controller (85) is connected to the two active switches G1 and G2 to switch them on or off. The charger (83) is connected to the secondary side of the transformer (82) through a rectification and filter circuit (86) to acquire a charging power. The battery set (84) is connected with an output terminal of the charger (83) and the switch circuit (81) to facilitate charging the battery set (84) through the charger (83). When the AC mains is out, the DC power of the battery set (84) supplies an square-wave AC power to the active power supply device (70) through the switching of the switch circuit (81).
When the AC mains normally supplies power, the UPS (80) is operated at line mode to let the AC mains directly supply power to the active power supply device (70). On the other hand, when the AC mains is out, the switch circuit (81) of the UPS (80) converts the DC power of the battery set (84) into an square-wave AC power to output through the transformer (82). With reference to FIGS. 11B and 11C for the generation method of the AC power signal in the form of square waves, the controller is instructed to alternately output two pulse width modulation (PWM) signals VG1, VG2 to two active switches G1 and G2, alternately switching the two active switches G1 and G2 on and off and outputting a square-wave AC power VO (as shown in FIG. 11A) induced and charged by the secondary side of the transformer. Whereas, if the square-wave AC power VO outputs to the active power supply device (70) and it is uncertain that the active power supply device (70) can still normally supply working power, the reason behind may be:
after the AC mains is out, the UPS enters the battery mode to output the square-wave AC power (as shown in FIG. 12A); when an off-time T/2 of two adjacent positive and negative square waves in a cycle T of the square-wave AC power VO (as shown in FIG. 12B) is greater than the hold-up time TH of the active power supply device (70), under such circumstance (as shown in FIG. 12C), the voltage of the output capacitor Cbulk of the active power supply device (70) has dropped below the low voltage threshold value VBL and can no longer output the working power (as shown in FIG. 12D). Therefore, the UPS (80) fails to meet the hold-up time demanded by the active power supply device (70) and thus is not feasible to support the operation of the active power supply device (70).